Question 1

Which of the following best describes the function of a regulatory gene in the lac operon?

Accepted Answer

It produces a repressor protein that can bind to the operator.

Question 2

In the trp operon, what is the role of the operator region?

Accepted Answer

It is the site where the repressor protein binds to block transcription.

Question 3

A mutation in the promoter region of the lac operon that prevents RNA polymerase from binding would most likely result in:

Accepted Answer

No transcription of the lac structural genes.

Question 4

Which of the following statements about the lac operon is correct?

Accepted Answer

The lac operon is repressed when lactose is absent.

Question 5

If a mutation causes the repressor protein of the trp operon to be nonfunctional, what would be the most likely outcome?

Accepted Answer

The trp operon would be constitutively expressed, regardless of tryptophan levels.

Question 6

Which of the following best explains the difference between inducible and repressible operons?

Accepted Answer

Inducible operons are usually off and can be turned on, while repressible operons are usually on and can be turned off.

Question 7

Suppose a bacterium has a mutation that prevents the lac repressor from binding to the operator. What would be the effect on the expression of the lac operon genes?

Accepted Answer

The lac operon genes would be constitutively expressed, even in the absence of lactose.

Question 8

Which of the following is the correct sequence of events for gene expression in the lac operon when lactose is present?

Accepted Answer

Lactose binds to the repressor, repressor detaches from operator, RNA polymerase transcribes structural genes.

Question 9

A scientist discovers a bacterial strain in which the trp operon is always off, even when tryptophan is absent. Which of the following is the most likely explanation?

Accepted Answer

A mutation in the operator prevents RNA polymerase from binding.

Question 10

Which of the following equations best represents the relationship between the rate of mRNA synthesis ($$R$$), the presence of an inducer ($$I$$), and the binding of the repressor ($$B) in an $$inducible operon?

Accepted Answer

$$R = k \cdot I \cdot (1 - B)$$

Question 11

In a clinical scenario, a patient is found to have a bacterial infection with a strain that cannot synthesize tryptophan. Which gene in the trp operon is most likely mutated?

Accepted Answer

One of the structural genes encoding tryptophan biosynthetic enzymes.

Question 12

Which of the following best describes the function of the enzyme $$eta-$$galactosidase in the lac operon?

Accepted Answer

It breaks down lactose into glucose and galactose.

Question 13

If the operator sequence of the lac operon is deleted, what is the most likely effect on gene expression?

Accepted Answer

The repressor cannot bind, so the operon is constitutively expressed.

Question 14

Which of the following is a key difference between the lac and trp operons?

Accepted Answer

The lac operon is inducible, while the trp operon is repressible.

Question 15

A researcher adds a molecule that mimics lactose to a culture of E. coli. What is the expected effect on the lac operon?

Accepted Answer

The mimic will bind to the repressor, causing it to release from the operator and allowing transcription.

Question 16

Which of the following best explains why the trp operon is considered a negative feedback system?

Accepted Answer

The presence of tryptophan represses the operon, reducing further tryptophan synthesis.

Question 17

In a real-world scenario, why might a bacterium benefit from having an inducible operon like the lac operon?

Accepted Answer

It allows the bacterium to conserve energy by only producing enzymes when lactose is present.

Question 18

Which of the following would most likely occur if a mutation caused the trp repressor to always bind to the operator, even in the absence of tryptophan?

Accepted Answer

The trp operon would be permanently repressed, and tryptophan biosynthesis would not occur.

Question 19

Given the following scenario: In a bacterial cell, both glucose and lactose are present. What is the expected state of the lac operon?

Accepted Answer

The lac operon is repressed due to catabolite repression by glucose.

Control of Gene Expression: The Operon Model

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